feat(sfm): unified deployment timeline (deprecate deployment_records)

Phase 4.  Rebuilds the seismograph "Deployment History" + "Timeline"
sections on the unit detail page as a single derived view computed from
three sources: unit_assignments (authoritative project/location windows),
unit_history (calibration/retirement/deployed state changes), and SFM
events overlaid per assignment window (count + peak PVS + last event).

Fixes the wonky-timeline symptoms: missing entries, duplicate/contradictory
rows, and no visibility into what the unit was actually doing during each
deployment window.

Backend:
- backend/services/deployment_timeline.py: new deployment_timeline_for_unit()
  helper.  Merges UnitAssignment rows (with SFM event overlay fetched
  concurrently via httpx), UnitHistory state-change rows (filtered to
  meaningful change_types and de-noised by dropping rows where
  old_value == new_value — there's noise in legacy audit log from
  record_history() being called on every save), and synthetic "gap"
  entries between assignments >= 1 day apart.  Sorts newest first.

- backend/routers/units.py: new GET /api/units/{unit_id}/deployment_timeline
  endpoint with optional include_events=false flag.

- backend/routers/project_locations.py: assign / unassign / swap /
  update endpoints now write UnitHistory rows on every assignment
  lifecycle event.  New change_types: assignment_created,
  assignment_ended, assignment_swapped, assignment_updated.  These
  surface in the unified timeline (where the assignment row itself
  shows the structural data; the audit row is filtered out to avoid
  double-rendering).  Closes a real gap — assignment changes were
  previously invisible to any audit consumer.

- backend/migrate_deprecate_deployment_records.py: non-destructive
  migration.  Adds deployment_records.deprecated_at column.  For each
  legacy row without a matching UnitAssignment, best-effort
  synthesizes one (with the free-text location_name preserved in
  notes).  Marks every processed row.  Idempotent.  DROP TABLE
  deferred to a follow-up release.

Frontend (templates/unit_detail.html):
- Removed legacy "Deployment History" card (with Log Deployment button)
  and the separate "Timeline" card.  Replaced with a single
  "Deployment Timeline" section.
- Three entry visual styles: assignment rows (orange dot, location +
  project link, event-overlay summary), gap rows (dashed outline, idle
  day count), and state_change rows (navy dot, friendly label, old →
  new value).  Active assignments get a green dot + "active" badge.
- Existing loadUnitHistory() and loadDeploymentHistory() functions kept
  as shims that delegate to loadDeploymentTimeline(), so modal-save
  callbacks that referenced them still trigger a refresh of the visible
  section.  Legacy function bodies preserved under _legacy_*_unused
  names for archeology; not called by anything.

Verified end-to-end:
- BE11529 timeline now shows 2 entries (active assignment with 24-event
  overlay + the deployed→benched state change), compared to the previous
  noisy mix that included 6 no-op state-change rows.
- Migration ran against real DB: 1 legacy row processed (had no
  project_id, marked deprecated without backfill).
- Assign / unassign / swap / edit now leave a paper trail in
  unit_history.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/routers/units.py

@@ -136,3 +136,37 @@ async def get_unit_events(
     )
     result["device_type"] = unit.device_type
     return result
+
+
+@router.get("/units/{unit_id}/deployment_timeline")
+async def get_unit_deployment_timeline(
+    unit_id: str,
+    include_events: bool = Query(True),
+    db: Session = Depends(get_db),
+):
+    """
+    Return a chronological deployment timeline for a unit.
+
+    Merges three sources:
+      1. unit_assignments — authoritative project/location deployments
+      2. unit_history — state changes (calibration, retirement, etc.)
+      3. SFM events — per-assignment overlay (count + peak PVS + last event)
+
+    Replaces the legacy /api/deployments/{unit_id} (which read the
+    deprecated `deployment_records` table) and the
+    /api/roster/history/{unit_id} timeline endpoint, unifying them into
+    a single derived view.
+
+    Gaps >= 1 day between consecutive assignments are surfaced as
+    synthetic "gap" entries.
+
+    Pass include_events=false to skip the SFM event overlay (saves N
+    HTTP calls; useful for fast text-only history dumps).
+    """
+    from backend.services.deployment_timeline import deployment_timeline_for_unit
+
+    return await deployment_timeline_for_unit(
+        db,
+        unit_id,
+        include_event_overlay=include_events,
+    )